This invention relates to programmable controllers and duplexed network systems. More particularly, the invention relates to duplexing methods for a network connected to a programmable controller.
Programmable controllers are being used as controlling devices for the factory automation set at a production factory (or a production site). Such a programmable controller (PLC) is comprised of a plurality of units, that is, it is formed by appropriately combining units of various types such as a power unit serving as a supply source of electric power, a CPU unit for carrying out the control of the PLC as a whole, an input unit for inputting signals from switches and sensors set at appropriate positions of production device or an equipment of the factory automation, an output unit for transmitting control outputs to actuators and a communication unit for connecting to a communication network.
The control by this CPU unit of a PLC is carried out by taking in signals inputted by the input unit into the I/O memory of the CPU unit (IN refresh), performing logical calculations based on a user program in a preliminarily registered user program language such as a ladder language (calculation), writing the results of the performed calculations in the I/O memory to thereby transmit them to the output unit (OUT refresh) and to thereafter cyclically repeat peripheral processes such as exchanging data with other PLCs on the communication network through the communication unit or with external apparatus through communication ports on the CPU unit. The IN refresh and the OUT refresh may be sometimes carried out summarily (I/O refresh).
As disclosed in Japanese Patent Publication Tokkai 2001-156818, for example, network systems are sometimes doubled, or duplexed, in order to improve system safety and reliability. According to this disclosure, an annular transmission system is formed with an optical cable arranged in an annular form such that nodes such as PLCs connected to this annular transmission route can be maintained in mutually connected conditions even if the transmission route is broken somewhere and hence a breakdown in communication can be prevented. Network reliability can thus be improved.
The PLC is connected to the network through the communication unit (the network unit) and is adapted to exchange data with the other nodes such as PLCs through this communication unit. Thus, interruptions of communication due to a breakage in the transmission line can be prevented by using such a duplexed transmission system with an annular transmission route but there still remains the problem that communication may become impossible for a node (such as a PLC) connected to a network through a communication unit because an abnormality occurs in the communication unit itself.
Still another problem of this prior art system is that it is not applicable to any other network formation because a network formation with an optical cable in an annular form is presupposed. In other words, the networks commonly used currently using wire cables and networks using bus-type wiring cannot be duplexed. Moreover, since an optical cable is used, the cost of the cable becomes greater than if a wire cable is used. Furthermore, even if an optical cable is arranged in an annular form, the network becomes disconnected if a breakage occurs at two or more places and communication can no longer be continued.